The objectives are to describe the ability of animals to process temporal cues, and to identify the minimum characteristics of an internal clock necessary to account for the facts of animal time perception. The specific aims are: (1) to identify the temporal code (absolute or relative); (2) to determine whether an animal learns a rule for controlling its internal clock, or learns many instances; (3) to describe the types of controls of the internal clock that animals can employ; (4) to determine whether or not animals deal with longer intervals by subdividing them; (5) to identify the independent stages of timing; and, (6) to determine the combination rule for time. Inferences regarding the internal representation of time of rats will be made from psychophysical choice procedures and discrete-trial, fixed-interval schedules of reinforcement.